A vacuum transport system provides shingled sheets across a stack prior to individually registering the sheets onto the stack. Shingling the sheets allows sheet and transport velocity and acceleration levels to be relatively low, and thus not stressful to transport drives and to the sheets. This allows an incoming sheet stream to be reliably stacked at a very high stack rate.
A basic finishing function for a production printer is a high capacity stacker. The purpose of the stacker is to compile printed sheets into a well-formed stack suitable to user end requirements, such as off-line finishing or bulk distribution. Current production printers are equipped with a high capacity stacker that produces a stack in which sheets can be optionally offset to one of two positions in the cross-process direction. This stacker design has proven effective and reliable at speeds of at least 110 ppm.
FIG. 1 shows a schematic of a conventional high capacity stacker. Sheets (unshown) enter from the left into the horizontal transport in area 1, pass through a mid transport in area 2 into a turn transport in area 3, after which the sheets are individually offset in the cross-process direction in area 4, and then pass onto a vacuum gripper transport (VGT) subsystem in area 5. The offset function may be performed via a nip pair similar to that used for print registration. An example of such an offset function can be found in U.S. Pat. No. 5,697,608 to Castelli et al., the disclosure of which is hereby incorporated herein in its entirety.
The conventional VGT transport consists of torso independently driven belt transport assemblies, VGT-1 and VGT-2, each having vacuum ports 240 (FIG. 2) and vacuum plenums 210 (FIG. 2) in order to successively acquire a leading edge of each sheet transported from offsetting nip 220 (FIG. 2) and then drag the sheet by its lead edge across the stack (right to left in the drawing) into a registration nip 230. At the registration nip 230, a series of scuffer belts 250 draw each lead edge up against a registration wall 260. The VGT thus acts much like a mechanical gripper system except that the gripping force is supplied solely by vacuum.
FIG. 2 shows a simplified view of a conventional VGT transport system 9200. Each VGT transport sub-assembly VGT-1 and VGT-2 has a multiplicity of belts spatially offset in the cross-process direction. The VGT-1 belts are interdigitated with the VGT-2 belts to enable sheets to smoothly transfer from VGT-1 to VGT-2. Each belt includes two sets of holes forming ports 240 located 180° apart from each other. When a set of holes 240 passes below the plenum areas 210 shown, vacuum will be transmitted from the plenum through the set of holes 240. If a sheet lead edge is aligned with the holes 240, the sheet will be acquired by the VGT-1 belts for transport by the belts. When the VGT-1 belt holes 240 pass out of the extreme left end of the first plenum zone 210, vacuum is no longer transmitted to the sheet. However, because the VGT-2 belts and plenum are sufficiently overlapped or abutting the VGT-1 belts and plenum, when the VGT-2 belt holes 240 pass the extreme right end of the second plenum 210, the sheet lead edge is acquired by the VGT-2 belts and transported further leftward. When the VGT-2 belt holes pass 240 out of the extreme left end of the plenum zone 210, vacuum is no longer transmitted to the sheet and it is released into the registration nip 230 where it is stacked against registration wall 260 by scuffer belts 250.